Ventilation Guide: PIV, Extractors and Condensation Control
Ventilation is the cure for most of North London's damp complaints — but the market sells boxes, not systems. Here's how each option works, what it costs, and which house needs which.
What is a PIV unit and does it work? Positive Input Ventilation (PIV) is a whole-dwelling ventilation system: a quiet fan unit (loft-mounted in houses, wall- or cupboard-mounted in flats) continuously introduces filtered air at a gentle rate, very slightly pressurising the home so stale, moisture-laden air is pushed out through natural leakage points. For chronically condensating homes it's typically the most effective single remedy available — condensation and mould reduction across every room, not just where a fan points — at £800–£1,500 installed (£800–£1,300 loft units, £900–£1,500 flat units) and pence per week to run. It works best alongside the rest of the toolkit: humidity-controlled extraction in wet rooms (£150–£400/room), trickle vents for background air (£20–£50 each), and moisture-habit basics.
Why ventilation became the problem
North London's older housing was ventilated by accident: open chimneys drawing air constantly, sash windows leaking at every joint, suspended floors breathing through airbricks. Twentieth- and twenty-first-century improvement — double glazing, draught-proofing, blocked flues, sealed extensions — removed that accidental system without installing a deliberate one, while modern living (daily showers, indoor drying, more occupants per dwelling) raised moisture production. The result is the sealed, condensating home: 10–15 litres of household water vapour daily with nowhere to go but the coldest surfaces, and black mould following — the full physics in the condensation and mould guide.
Ventilation strategy is therefore restoration: putting back, deliberately and efficiently, the air change the building lost. The toolkit runs from £20 vents to whole-house systems, and matching it to the dwelling is the skill.
Extraction: the wet-room workhorses
Bathrooms and kitchens produce the moisture spikes; extraction intercepts them at source. What separates working extraction from the whirring token most homes have: humidity-controlled units that run until the air is actually dry (not 15-minute timers that quit mid-condensation), adequate extract rates for the room, ducting that reaches outside — not a loft void, where 'extraction' becomes loft condensation — and short, smooth duct runs that don't strangle the fan. Installed cost £150–£400 per room; continuous-running dMEV variants suit rentals where switch-off is the failure mode.
Trickle vents (£20–£50 per window, retrofittable to most frames) supply the background air change that makes extraction effective — fans need makeup air — and stop sealed rooms stagnating. Unfashionable, near-invisible, and collectively transformative in a sealed house.
PIV: the whole-house answer
Where condensation is dwelling-wide — mould in bedrooms, streaming windows throughout, the classic sealed-conversion or improved-semi pattern — room-by-room measures chase symptoms. PIV changes the whole building's air: the unit draws air (from the loft in houses, gaining free solar warmth much of the year; from outside via a wall unit in flats), filters it, and delivers it continuously to a central hallway point at a rate that turns the home's air over gently but constantly. Moisture-laden air exits through the building's natural leakage; relative humidity falls; surfaces stop reaching dew point; mould loses its habitat — typically within weeks.
Costs and practicalities: £800–£1,300 installed for loft-mounted house units, £900–£1,500 for flat versions; running costs pence per week; maintenance a filter change every few years; slight background airflow at the diffuser the only occupant-noticeable effect. Suitability caveats: the dwelling needs somewhere for air to leave (hermetic new-builds suit MVHR instead), and PIV complements rather than replaces wet-room extraction. Our ventilation solutions service covers assessment and installation.
Choosing a strategy — house by house
Sealed interwar semi or improved family house (the Finchley/Barnet pattern): trickle vents + humidity-controlled wet-room extraction + PIV for whole-house cases — £500–£2,000 total, transformative. Converted period flat (the Islington/West Hampstead pattern): ducted extraction from internal wet rooms + wall-mounted PIV where condensation is flat-wide — £300–£1,800. Rental property: continuous-running dMEV extraction + background ventilation as standard spec — occupant-proof, compliance-friendly (see the landlord guide). Airtight new-build or deep retrofit: MVHR (mechanical ventilation with heat recovery) — ducted supply and extract with heat exchange, £3,000–£8,000+ in retrofit, the right answer where airtightness makes PIV's leakage-path assumption fail.
Sequence for any home: fix the moisture habits (free), make wet-room extraction real (£150–£400/room), restore background air (£20–£50/vent), then escalate to PIV if condensation persists dwelling-wide. The ventilation audit checklist walks the assessment room by room.
Related services, guides and tools
Ventilation Solutions (PIV)
The service this guide supports — diagnosis-first, with independent qualified providers.
View the serviceCondensation Control
The service this guide supports — diagnosis-first, with independent qualified providers.
View the serviceLoft Condensation
The service this guide supports — diagnosis-first, with independent qualified providers.
View the serviceThe Complete Guide to Condensation and Mould
Related reading from the Knowledge Centre.
Read the guideProperty Ventilation Audit Checklist
Free, instant and anonymous — a two-minute head start on your answer.
Use the toolCondensation Risk Calculator
Free, instant and anonymous — a two-minute head start on your answer.
Use the toolReferences & further reading: BRE ventilation guidance · Building Regulations Approved Document F · Property Care Association (PCA) · NHS damp and mould guidance. This guide is general information for North London property owners, not professional advice for a specific building — a survey provides that.
Ventilation Guide FAQs
For whole-dwelling condensation, it's typically the most effective single measure available: continuous filtered input lowers humidity everywhere, surfaces stop reaching dew point, and mould loses its conditions — usually within weeks. It complements wet-room extraction rather than replacing it.
£800–£1,300 for loft-mounted house units, £900–£1,500 for wall- or cupboard-mounted flat versions, installed. Running costs are pence per week; maintenance is a filter change every few years.
Good units run near-silently on low settings; the noticeable effect is slight air movement at the hallway diffuser. Loft units temper incoming air with free loft warmth; heated models exist for the coldest weeks.
Usually one of four: it's timer-based and quits before the air is dry, undersized, ducted into a void rather than outside, or starved of makeup air in a sealed room. Humidity-controlled units, real ducting and a trickle vent transform performance.
PIV pushes filtered air in and relies on natural leakage paths out — ideal for older, naturally leaky stock. MVHR ducts both supply and extract with heat recovery — right for airtight new-builds and deep retrofits, at several times the cost. Building airtightness picks the winner.
No — ventilation cures condensation. Structural damp needs its structural remedy (DPC works, defect repair), though drying any treated wall benefits from good air change. Mechanism first: the diagnostic tool or a survey tells you which problem you actually have.
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